Fibroblastic reticular cells (FRCs), one of the major populations of non-hematopoietic stromal cells in lymph node (LNs), secrete extracellular matrix components to form a dense reticular network and lymph-draining conduit system. The T cell zone is delineated by FRCs, forming a scaffold to provide essential guidance cues to immune cells. FRCs orchestrate immune cell migration via expression of CCL19 and CCL21, as well as adhesion molecules, integrins, and glycoproteins. Beyond migration, FRCs maintain nave T cell homeostasis and they have the capacity to impose antigen-specific deletional tolerance, with direct presentation of viral and self-peptides to nave CD8+ T cells. The timing of deletional events in these studies, whether an inevitable outcome of an FRC-mediated activation signal to nave T cells, or a result of subsequent feedback to the FRC from the activated T cell, is unknown, as are its driving molecular mechanisms. Additionally, in the context of immune response, T cells are usually activated by dendritic cells (DCs) while in direct contact with the FRC network, therefore any effect of FRCs on activated T cells is highly relevant. Our recent studies suggest that FRCs can acquire suppressive function at different stages in the T cell response; this newly appreciated function of FRCs appears to control the expansion of potentially pathogenic T cells via a NOS2-dependent mechanism. This project aims to further define the molecular and physical basis of the interactions between activated T cells and FRCs within lymph nodes. Despite recent advances indicating that stromal determinants of secondary lymphoid organs exhibit complex regulatory roles during immune responses, our knowledge of the stromal niche and how it impacts T cell immunity and tolerance is limited. The proposed studies will define the molecular crosstalk that culminates in CD8 T cell suppression by FRCs. They will also establish the influence of FRCs on the positioning and motility of activated T cells Finally, these studies will determine the capacity of FRCs to present lymph-borne antigen to CD4 T cells. The results of these studies will elucidate the molecular and physical interactions between lymph node FRCs and newly activated CD8 and CD4 T cells, and the role of FRCs in patrolling the lymph by sampling antigens from the conduit system, and the impact of inflammation on these processes.